Every breed of dog has a characteristic temperament.
This is not to say that there aren't exceptions, which of course is always the rule.
But barring odd accidents, one can safely assume such things like, Labradors are good with children, even when the child in question is attempting to murder the dog, the dog usually displays surprising prudence in dealing with the situation - he does not attempt to swallow the kid whole, but rather believes in teaching with a gentle hand.
Precisely what the child's parents would prefer and in fact, what is good for the dog in the long run.
Similarly, it can be said of Chihuahuas that they are not good baby-sitters.
They are temperamental, yes, but that is only a minor defect.
The real issue is that they are so small and fragile that were a child to attempt murdering one, chances are that it would succeed with little or no damage to itself.
And this would undoubtedly be traumatic for the child.
In the event that the dog does survive, however, if it's a true Chihuahua, it will probably not spare a chance to murder the child in return, which again would be traumatic for the child.
We don't know what causes the Chihuahua trauma and what doesn't so we can't discuss that, but this much is certain, the Chihuahua isn't an animal to be experimented with.
It usually is like a soppy lover, overly anxious for attention and petting from its master and jealous of anything that seems like is could get in the way, including and perhaps particularly children for in little people it sees competition, which it usually dislikes.